Cervical plate fixation system

ABSTRACT

A bone fixation assembly comprises a plate having a top surface and a bottom surface and defining at least one bone screw bore, the bore including a circumferential rib situated between the top and bottom surfaces of the plate and at least one scallop interrupting the circumferential rib. A bone engaging fastener is provided for each bore, the fastener having a shank defining bone engaging threads and an enlarged head. The head defines threads for threaded engagement with the circumferential rib, with a lead thread configured to pass through a scallop to allow the threads to engage the circumferential rib.

PRIORITY CLAIM AND REFERENCE TO RELATED APPLICATION

The present application is a continuation of and claims priority toapplication Ser. No. 12/504,408, filed on Jul. 16, 2009, which issued onSep. 16, 2014, as U.S. Pat. No. 8,834,536.

BACKGROUND

The present invention relates to fixation systems for use in stabilizingand immobilizing spinal segments, particularly in the cervical spine.

Bone fixation devices are useful for promoting the proper healing ofinjured or damaged spinal motion segments caused by trauma, tumorgrowth, degenerative disc disease or other spinal pathologies that maynecessitate permanent immobilization. These fixation devices aretypically used to immobilize the injured/damaged bone or motion segmentsto ensure the proper growth of new osseous tissue between the damagedsegments.

One type of fixation system utilizes an osteosynthesis plate, morecommonly referred to as a bone plate that can be used to immobilizeadjacent skeletal parts such as vertebral bones. Typically, the fixationplate is a rigid metal or polymeric plate positioned to span bones orbone segments that require immobilization with respect to one another.The plate is fastened to the respective bones using anchors, such asbone screws, so that the plate remains in contact with the bones andfixes them in a desired position. Cervical plates, for instance, can beuseful in providing the mechanical support necessary to keep vertebralbodies of the cervical spine in proper position, and in bridging aweakened or diseased area such as when a disc, vertebral body or spinalfragment has been removed. These cervical plates usually include a rigidbone plate having a plurality of screw openings in the form of holes orslots that allow for freedom of screw movement. The bone plate is placedagainst the damaged vertebral bodies and bone screws are used to securethe bone plate to the spine, usually with the bone screws being driveninto the vertebral bodies. Cervical bone plates are typically placedanteriorly, although posterior or transverse fixation is also known.

Because the cervical spine is routinely subject to mechanical loadswhich cycle during movement, one of the primary concerns is the risk ofscrew pullout. This is of particular concern in the cervical regionbecause of the critical vessels that abut the anterior surfaces of thecervical spine. Screw pullout often occurs when the cylindrical portionof the bone which surrounds the inserted screw fails. A bone screw whichis implanted perpendicular to the plate is particularly weak because theregion of the bone which must fail for pullout to occur is only as largeas the outer diameter of the screw threads. It has been found that forpullout to occur for a pair of screws which are angled relative to eachother and to the plate, the amount of bone which must fail increasessubstantially as compared to pairs of screws which are implanted inparallel along the axis that the loading force is applied. It has,therefore, been one goal of those in the art to provide a cervicalscrew-plate assembly that permits the screws to be entered into thevertebral body at angles other than 90 degrees.

As mentioned above, a great concern with screws being implanted in theanterior portion of the cervical spine is that there are importantinternal tissue structures which may be damaged by a dislocated screw.In the cervical spine, the esophagus is located directly in front of theanterior surface of the vertebral body, and therefore, in potentialcontact with an implanted cervical plate. Because screw pulloutrepresents one of the largest risks of esophageal perforation, it hasbeen a further goal object of those in the art to produce a cervicalscrew-plate design that prevents the screw from separating from theplate, even if the bone holding the screw fails.

One typical screw-plate design provides angled holes for insertion ofthe bone screw. This typical design, as represented by the Orion®Anterior Cervical Plate System of Sofamor Danek, further includes anadditional threaded hole disposed between pairs of bone screw holes sothat a corresponding set screw may be inserted to lock the bone screwsto the plate. Although the Orion® system achieved certain advantagesover prior cervical screw plate assemblies, one drawback is that a givenplate can accommodate only one screw angular orientation per hole. Thisis undesirable, in that physicians often must inspect the vertebralbodies during the implantation procedure before making the decision asto which screw-in angle is the ideal. While providing a variety ofplates having different angle bone screw holes is possible, thecomplexity and expense of providing a full spectrum of plates availablein the operating room for the surgeon to choose from is undesirable.

In order to address the concerns of these screw-plate systems, othersystems have been developed that permit polyaxial coupling of the screwto the plate, whereby a single plate is compatible with a wide range ofscrew-in angles. In typical systems of this type, the head of the bonescrew is spherical to match a spherical surface on the holes formed inthe cervical plate. The bone screw can thus be oriented at a wide rangeof angles relative to the plate. In other systems, the bone screw isseated within an insert that is mounted within the screw holes in theplate, wherein the insert permits the variable angle placement of thescrew. In both systems, a separate component, such as a set screw, isrequired to lock the assembly in position and prevent back-out of thebone screw from the plate.

There remains a need for an orthopedic screw plate assembly whichprovides an effective mechanism for engaging a bone screw to the plateat any desired angular orientation. It is desirable that the assemblyrequire as few parts as possible to simplify the surgeon's task duringimplantation into the cervical spine.

SUMMARY

In one embodiment, a bone fixation assembly comprises a plate having atop surface and a bottom surface and defining at least one bore betweenthe surfaces adapted to receive a bone engaging fastener therethrough.Each bore includes an inner surface and a circumferential rib projectingtherefrom the inner surface and situated between the top and bottomsurfaces of the plate. Each bore further defines at least one scallopinterrupting the circumferential rib.

The fixation assembly further comprises a bone engaging fastenercorresponding to each of the bores, the fastener having a shank definingbone engaging threads and an enlarged head. The head defines lockingthreads for threaded engagement with the circumferential rib. Thelocking threads include a lead thread that is configured to pass througha scallop to allow the remaining threads to engage the circumferentialrib.

In one specific embodiment, each bore defines at least three scallopssubstantially evenly distributed around the circumference of thecircumferential rib. The scallops allow the bone engaging fastener toachieve different angles relative to the plate and at differentazimuthal positions based on which scallop the lead screw engages first.

In a further embodiment, the plate is a substantially rectangular platesized and shaped for fixation between at least two adjacent cervicalvertebrae. The plate includes four bone screw bores, one each situatedat each corner of the substantially rectangular plate. Four boneengaging fasteners are provided for engagement within a correspondingone of the four bores. In this embodiment, each of the four bores issubstantially identically configured and each of the four bone engagingfasteners is substantially identically configured so that each bonefastener can adopt variable angular orientations relative to the plateas dictated by the cervical anatomy.

In one feature, the circumferential rib defines an inner edge and thebore further defines a scallop run-out extending tangentially from ascallop to the inner edge. The scallop run-out merges substantiallytangentially with the inner edge. In one aspect, the run-out forms asharp corner with the next adjacent scallop. The sharp corner preventsthe head of the fastener from backing out of the bore. In a specificembodiment, each bore defines at least three scallops substantiallyspaced apart 120 degrees around the circumference of the circumferentialrib with each scallop including a corresponding scallop run-outextending from the scallop and merging with the inner edge about 80degrees from the scallop.

According to a further aspect, a method is provided for engaging afixation plate to a bone that comprises the step of providing a fixationplate having a top surface and a bottom surface and defining at leastone bore between the surfaces, the bore including an inner surface and acircumferential rib projecting from the inner surface and situatedbetween the top and bottom surfaces of the plate, and the bore furtherdefining at least one scallop interrupting the circumferential rib. In afurther step, a bone engaging fastener is provided corresponding to eachof the at least one bore, the fastener having a shank defining boneengaging threads and a head, the head defining threads for threadedengagement with the circumferential rib, the threads including a leadthread configured to pass through the scallop to allow the threads toengage the circumferential rib.

The method further comprises positioning the fixation plate on the bonewith the bottom surface contacting the bone, passing the shank of thebone engaging fastener through the bore, driving the shank of the boneengaging fastener into the bone at any angle relative to the fixationplate until the lead thread of the head of the fastener is within thebore, and rotating the fastener so that the lead thread passes through ascallop in the bore. The fastener is rotated until the head of thefastener is fully seated within the bore.

The present invention provides an improved bone fixation assembly, andmore particularly an assembly that allows engagement of a bone fastenerat multiple angular orientations. The fixation assembly further providesmeans for locking the fastener to the bone plate so that the assemblydoes not loosen over time.

DESCRIPTION OF THE FIGURES

FIG. 1 is a top view of a cervical fixation plate according to oneembodiment.

FIG. 2 is a side cross-sectional view of the plate shown in FIG. 1,taken along line 2-2 as viewed in the direction of the arrows.

FIG. 3 is a top view of an end portion of the plate shown in FIG. 1.

FIG. 4 is an enlarged cross-sectional view of the plate shown in FIG. 2.

FIG. 5 is an enlarged top perspective view of a screw bore of the plateshown in FIG. 1.

FIG. 6 is an end perspective partial cut-away view of the plate shown inFIG. 1.

FIG. 7 is a perspective view of the plate shown in FIG. 1.

FIG. 8 is a top view of an alternative plate incorporating the presentinvention,

FIG. 9 is a top view of another alternative plate incorporating thepresent invention.

FIG. 10 is a side perspective view of a bone screw according to afurther embodiment disclosed herein.

FIG. 11 is an enlarged view of a portion of the head of the bone screwshown in FIG. 10.

FIG. 12 is a side view of an alternative bone screw.

FIGS. 13( a)-(c) are partial cross-sectional views of a screw bore ofthe cervical plates disclosed herein with a bone screw engaged atdifferent angles relative to the plate.

DESCRIPTION OF THE EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and described in the following written specification. It isunderstood that no limitation to the scope of the invention is therebyintended. It is further understood that the present invention includesany alterations and modifications to the illustrated embodiments andincludes further applications of the principles of the invention aswould normally occur to one skilled in the art to which this inventionpertains.

Referring to FIGS. 1-7, a cervical fixation plate 10 is depicted thatincludes a central cut-out 14 and four bone screw bores 16 defined atthe corners of the generally rectangular plate. The lower surface 12 ofthe plate is configured or contoured for minimal prominence when theplate is mounted to adjacent cervical vertebrae. Similarly, the uppersurface 11 is fashioned to maintain a smooth profile to avoid irritationto the soft tissue surrounding the implanted plate. The length andcurvature of the plate may be configured in a known manner forimplantation at a particular cervical level. Similarly, the location ofthe bone screw bores 16 may be in accordance with known standards, witha primary object being to locate the bone screws for solid engagement inthe cervical vertebral body. In one embodiment, the longitudinal axes ofthe screw bores are parallel to each other, rather than expressly to thetop or bottom surfaces 11, 12 of the plate. Thus, in one specificembodiment, the top surface 11 is gently curved, while the axes of thescrew bores 16 are offset by about five degrees relative to a normalaxis to the top surface. This angular offset may be inboard relative tothe normal axis.

The fixation plates 10′ and 10″ shown in FIGS. 8-9 further illustratethe configurations of fixation plates for instrumenting variousvertebral levels, particularly in the cervical spine. The plate 10includes two sets of bone screw bores for engaging two vertebral bodies,while the plate 10′ includes three screw bore pairs for engaging acorresponding number of vertebrae. Likewise, the four sets of screwbores in the plate 10″ permits fixation to four vertebral bodies.

Turning to FIGS. 3-6, details of the plate 10 are shown, and moreparticularly details of the screw bores 16. In accordance with onefeature of the invention, the screw bores are configured to beself-locking at a range of angles, perpendicular and non-perpendicular,to the bottom surface 12 of the plate. The screw bores include an innersurface adapted to receive a bone screw therethrough, in which the innersurface includes an upper portion 18 extending from the top platesurface 11 and a lower portion 20 extending upward from the bottom platesurface 12. The two portions are separated by a rib element 22projecting from the inner surface having an upper surface 23 angledupward toward the upper portion 18 and a lower surface 24 angleddownward toward the lower portion 20. The two surfaces 23, 24 convergeto an inner edge 25 that has a thickness t. In the preferred embodiment,this thickness t is non-zero, yet small enough to fit within the threadpitch of a bone screw (such as screw 40 in FIG. 10) engaged therein, asexplained in more detail herein. In a specific embodiment, the upper andlower surfaces 23, 24 subtend an angle of 60 degrees to correspond tothe pitch of the bone screw thread.

In the illustrated embodiments, the upper and lower portions 18, 20 aredepicted as having the same depth. In these embodiments, the rib element22 is thus oriented at mid-thickness of the plate. In alternativeembodiments, the rib element 22 may be offset toward one or the other ofthe top and bottom surfaces 11 and 12 of the plate 10. In addition, theupper and lower portions are shown as cylindrical, although otherconfigurations are contemplated, such as conical or spherical.

In one feature of the invention, the rib element 22 includes a number ofscallops 28, as best seen in FIGS. 3 and 5. The scallops 28 aregenerally defined by circular cuts through the rib element 22 with theaxis of the scallops running generally parallel to the axis of thecorresponding screw bore 16. The scallops 28 include a linear scalloprun-out 29 that merges to the inner edge 25 of the rib element. With thescallop runout, the radius of the inner edge 25 thus becomes non-uniformaround the circumference of the rib element. Thus, the inner edge 25follows a constant radius from a scallop corner 30 at each scallop forabout one-third of the circumferential distance between successivescallops, for a screw bore having three such scallops. The scalloprun-out extends generally tangentially from the scallop diameter andmerges tangentially with the inner edge of the rib, as best seen in FIG.3. The scallop run-out 29 is oriented in the direction of tightening athreaded fastener, which in the embodiment illustrated in FIG. 3 is inthe clockwise direction. In other words, the point at which the scalloprun-out merges with the inner edge 25 of the rib element 22 is offsetclockwise from the origin of the run-out at the scallop 28.

By way of example, in one specific embodiment, the upper and lowerportions 18, 20 of the screw bores 16 have a diameter of 5.40 mm. Thescallops 28 are defined at a radius of 0.60 mm, with the center axisbeing aligned with the inner diameter of the inner edge 25, which in thespecific embodiment is 4.20 mm. In this embodiment, three scallops areprovided that are circumferentially offset by 120 degrees. Thus, eachscallop run-out 29 extends tangentially from the scallop diameter andcontinues clockwise about two-thirds of the circumferential offsetbetween successive scallops, or about 80 degrees from the originatingscallop.

In the illustrated embodiments, each screw bore 16 includes threescallops 28 uniformly spaced around the circumference of the bore. Itcan be appreciated that other numbers of scallops may be provided ineach screw bore, with different numbers of scallops in each bore of agiven plate. As explained herein, the scallops permit different angularorientations of a bone screw relative to the plate, with the number oforientations being a function of the number of scallops.

Turning to FIGS. 10-11, details can be discerned of one embodiment of abone screw adapted to engage the screw bores 16 of the cervical plate10. The bone screw 40 includes a head 42 which may including a driverecess 44 configured to accept a standard driving tool, such as a hex orTORX® driver. The shank 46 of the screw includes bone-engaging threads48, and may incorporate a self-tapping tip 49. The bone engaging threadsmay have a variety of configurations.

The length of the shank 46, as measured from below the head 42, isadapted to the particular bone in which the screw is to be engaged.Thus, in one specific embodiment, a cervical screw can be provided inlengths of 12.0 or 14.0 mm, with the head having a length of 3.5 mm. Thelength of the shank 46 is thus 8.5 or 10.5 mm. Thus, the bone engagingthreads 48 will engage the vertebral body to a minimum depthcorresponding to length of the shank. Depending upon the depth ofengagement of the head 42 of the bone screw 40 within the cervical plate10, the shank may be embedded deeper into the bone, but no greater thanthe total length of the screw less the thickness of the plate.

The head 42 of the screw 40 incorporates a feature adapted to engage therib element 22 of the bone plate 10. In particular, the head 42incorporates a gradually decreasing diameter from the upper surface 43to the intermediate portion 45 between the head and the shank 46. Thediameter at the upper surface 43 is less than the diameter of the upperportion 18 of the screw bores 16 but greater than the inner diameter ofthe inner edge 25 of the circumferential rib element 22. The diameter ofthe head 42, or more specifically the root diameter of the lockingthreads 50 described below, decreases to a diameter that is slightlyless than the inner diameter of the inner edge 25. (Of course, the outerdiameter of the bone engaging threads 48 is less than the inner diameterof the rib element to permit introduction of the shank 46 through thescrew bore 16.)

The head 42 includes locking threads 50 that are adapted to engage therib element 22 of the screw bore. In one embodiment, the root diameterof the threads is tapered inward from the upper surface 43 of the headto the intermediate portion 45. The outer diameter of the threads isparallel to the root diameter, with the exception of the lead thread 51.The lead thread 51 has a thread height that is about half the threadheight of the remainder of the locking threads 50 to facilitateintroduction of the locking threads into the screw bore 16. In oneembodiment, the thread faces 52 subtend an angle different from thesubtended angle of the upper and lower surfaces 23, 24 of the ribelement 22. Thus in one specific embodiment, the thread faces 52 are atan angle of 50 degrees relative to the longitudinal axis of the screw tosubtend an 80 degree angle.

In an alternative embodiment shown in FIG. 12, the head 42′ of the bonescrew 40′ may be spherical. The shank 46′ may be configured like theshank 46 just described for engagement with the cervical bone. Thelocking threads 50′ thus follow the spherical contours of the head 42′.The thread faces in this embodiment may subtend an angle that is thesame as the subtended angle of the rib element 22 surfaces. The lockingthreads may also be double threads starting 180 degrees apart.

As depicted in FIGS. 13( a)-(c), the locking threads can engage the ribelement 22 in different ways to achieve different angular orientationsrelative to the plate 10. As shown in FIG. 13( a), the screw 40 can bepositioned substantially perpendicular to the plate by threading thescrew directly into the opening 16 and through the inner diameterdefined by the inner edge 25 of the rib element 22. In this instance,the root of the uppermost thread 50 a spans the upper and lower surfaces23, 24 of the rib element 22. T

In FIG. 13( b), the screw 40 is oriented at an angle relative to theplate, which in the illustrated embodiment is about eight degrees. Inthis orientation, the uppermost thread 50 a contacts the upper surface23 of the rib element 22 while the next thread 50 b contacts the lowersurface 24 of the rib element at a circumferentially offset location. Inother words, while the thread 50 a contacts the upper surface of the ribadjacent one scallop, the thread 50 b will contact the lower surface ofthe rib adjacent another scallop.

In order to achieve a larger angular orientation, the first thread 50 acontacts the upper surface 23 of the rib element 22 at a sharper angle,and even dig into the rib element. The second thread 50 b also engagesthe upper surface 22 at a sharper angle so that the next thread 50 c maydig into the lower surface 24 of the rib element. In a specificembodiment, the screw 40 can thus achieve an angle of fifteen degreesrelative to the plate.

In one aspect of the illustrated embodiments, one side of the lockingthreads 50 provides a typical peak-to-valley mating, as exemplified bythe engagement of the uppermost thread 50 a with the lower surface 24 ofthe rib element 22. It can be appreciated that this same peak-to-valleyengagement occurs even if the screw 40 is not fully threaded into thescrew bore as depicted in the figures. For example, if the screw isbacked out one turn, the second thread 50 b will engage the lowersurface with the uppermost thread 50 a contacting the upper surface ofthe rib element. In this configuration, the rib element—the peak—willfit within the thread pitch—the valley.

However, the thread engagement 180 degrees offset from thepeak-to-valley engagement is an interference fit with the rib element22. For instance, in the arrangement shown in FIG. 13( a), the secondthread 50 b will dig into the rib element 22 as the screw 40 is driveninto the screw bore. The presence of the scallops 28 and scalloprun-outs 29 allow the locking threads to be initially threaded into thebore as the lowermost thread passes through one of the scallops.Moreover, the scallops allow the screw 40 to be driven into the screwbore in any of the orientations shown in FIGS. 13( a)-(c). In generalterms, the screw 40 will adopt the angle that the lowermost thread ofthe locking threads enters the first scallop.

This feature of the present invention allows the surgeon to drive thebone screw at the optimum angle for maximum engagement and fixationwithin the vertebral bone. During implantation, the screw bore 16 of thecervical plate 10 initially acts as a guide for placing the bone screw.The bone engaging threads 48 are driven into the vertebral bone at anydesired angle. As the bone engaging threads are driven to their fullestextent into the bone, the locking threads contact the bone plate, andparticularly the rib element. For a three scallop configuration, thescrew 40 need only be rotated a maximum of 80 degrees before thelowermost thread will reach a scallop 28. The scallop and the scalloprunout 29 allow the lowermost thread to fully engage the rib element 22so that continued rotation of the screw will draw the successive threadsinto engagement without risk of being dislodged at a downstream scallop.

Once the locking threads have been threaded to their fullest extent intothe screw bore, the interface becomes self-locking In particular, thescallop corner 30 (see FIG. 5) formed at the interface between eachscallop and the full diameter portion of the rib element prevents thescrew from backing out or counter-rotating. In particular, anycounter-rotation will cause the scallop corner 30 to dig into thelocking threads 50 of the screw 40. It can be appreciated that the ribelement 22 is not configured as a screw thread, but instead as a ringembedded within the screw bore. Consequently, when the threads of thescrew are counter-rotated, they are not following a normal threadcontour, but must instead immediately contact the scallop corner 30 ateach scallop 28.

It can thus be appreciated that the scallops 28 provided in the screwbore 16 provide multiple benefits. One significant benefit is that thescallops allow the bone screw to adopt a range of non-perpendicularangular orientations without the need for additional hardware found inprior screw-plate designs. Another benefit is that there is no need forforming female threads within the plate, which typically requires athicker plate. Thus, with the single rib configuration of the presentinvention, the cervical plate 10 may be thinner than prior platedesigns. A thinner plate means a lower profile and lower prominence,which in turn avoids soft tissue irritation that can often accompanyfixation plates. A further benefit is that the same structure thatfacilitates driving the bone screw in at multiple angles also provides aself-locking or anti-backout features, again without the need foradditional hardware founding prior screw-plate designs.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same should be considered asillustrative and not restrictive in character. It is understood thatonly the preferred embodiments have been presented and that all changes,modifications and further applications that come within the spirit ofthe invention are desired to be protected.

The bone plate and screw embodiments described herein have been for usein fixation of the cervical spine. It is understood that the sameprinciples may be employed at other locations in the spine, such as inthe lumbar spine, and for other types of fixation. For instance, theembodiments described herein may be modified for use in compression ofbone fractures or immobilization of joints, such as the bones and jointsof the hands and feet. The variable angular orientations and theself-locking features of the described embodiments may be implemented ina variety of environments for fixation of a plate to a bone. It isfurther contemplated that these features may be implemented in thefixation of plate segments that may be used to fasten an elongated rod,for instance, to a bone.

What is claimed is:
 1. A bone fixation assembly comprising: a platehaving a top surface and a bottom surface and defining at least one borebetween said surfaces, said bore including an inner surface and only onecircumferential rib projecting outwardly from said inner surface towardthe interior of said bore substantially continuously around said innersurface and situated between said top and bottom surfaces of said platesuch that the circumferential rib is spaced apart from the top surfaceand the bottom surface, said bore further defining at least one scallopinterrupting said circumferential rib only once; and a bone engagingfastener corresponding to each of said at least one bore, said fastenerhaving a shank defining bone engaging threads and a head, said headdefining helical locking threads for threaded engagement with saidcircumferential rib, said helical locking threads including a leadthread configured to pass through said scallop to allow said helicallocking threads to engage said circumferential rib.
 2. The bone fixationassembly according to claim 1, wherein said bore defines three scallopssubstantially evenly distributed around the circumference of saidcircumferential rib.
 3. The bone fixation assembly according to claim 1,wherein said at least one bore includes four substantially identicallyconfigured bores.
 4. The bone fixation assembly according to claim 1,wherein: said plate is a substantially rectangular plate sized andshaped for fixation between at least two adjacent cervical vertebrae;said at least one bore includes four bores, one each situated at eachcorner of said substantially rectangular plate; and four bone engagingfasteners are provided for engagement within a corresponding one of saidfour bores.
 5. The bone fixation assembly according to claim 4, whereineach of said four bores is substantially identically configured and eachof said four bone engaging fasteners is substantially identicallyconfigured.
 6. The bone fixation assembly according to claim 1, wherein:said circumferential rib defines an inner edge; and said bore furtherdefines a scallop run-out extending tangentially from a scallop to saidinner edge.
 7. The bone fixation assembly according to claim 6, whereinsaid scallop run-out merges substantially tangentially with said inneredge.
 8. The bone fixation assembly according to claim 6, wherein saidbore defines three scallops substantially evenly spaced apart 120degrees around the circumference of said circumferential rib, eachscallop including a corresponding scallop run-out extending from saidscallop and merging with said inner edge substantially 80 degrees fromthe tangential intersection of said run-out with said scallop.
 9. Thebone fixation assembly of claim 1, wherein said shank of said boneengaging fastener is sized to pass freely through said circumferentialrib.
 10. The bone fixation assembly of claim 1, wherein said boredefines a sharp corner between said at least one scallop and saidcircumferential rib.
 11. The bone fixation assembly according to claim1, wherein said circumferential rib is at a non-perpendicular anglerelative to a longitudinal axis of said bore extending between said topand bottom surfaces.
 12. The bone fixation assembly according to claim1, wherein said at least one scallop is defined by circular cuts throughsaid rib with an axis of the scallop extending between said top andbottom surfaces and running generally parallel to a longitudinal axis ofsaid bore extending between said top and bottom surfaces.
 13. The bonefixation assembly according to claim 6, wherein: said inner edge definesan inner diameter; and said one scallop run-out merges tangentially withsaid inner diameter.
 14. The bone fixation assembly according to claim6, wherein said one scallop run-out extends tangentially from theassociated scallop in a clockwise direction.
 15. The bone fixationassembly according to claim 14, wherein said bore defines a sharp comerbetween said at least one scallop and said circumferential ribcounterclockwise from said scallop run-out.
 16. A bone fixation platecomprising: a top surface and an opposite bottom surface; and at leastone bore defined between said surfaces, said bore including an innersurface and only one circumferential rib projecting outwardly from saidinner surface toward the interior of said bore substantiallycontinuously around said inner surface and situated between said top andbottom surfaces of said plate such that the circumferential rib isspaced apart from the to surface and the bottom surface, saidcircumferential rib defining an inner edge radially offset from saidinner surface toward the interior of said bore, said bore furtherdefining at least one scallop interrupting said circumferential rib andone scallop run-out associated with each scallop extending tangentiallyfrom the associated scallop to said inner edge.
 17. The bone fixationplate according to claim 16, wherein said bore defines three scallopssubstantially evenly spaced apart 120 degrees around the circumferenceof said circumferential rib, each scallop including a correspondingscallop run-out extending from said scallop and merging with said inneredge substantially 80 degrees from the tangential intersection of saidrun-out with said scallop.
 18. The bone fixation plate of claim 16,wherein said bore defines a sharp corner between said at least onescallop and said circumferential rib.
 19. The bone fixation plateaccording to claim 16, wherein said at least one scallop is defined bycircular cuts through said rib with an axis of the scallop extendingbetween said top and bottom surfaces and running generally parallel to alongitudinal axis of said bore extending between said top and bottomsurfaces.
 20. The bone fixation assembly according to claim 16, wherein:said inner edge defines an inner diameter; and said one scallop run-outmerges tangentially with said inner diameter.